Optical fiber cable having fiber in metal tube core with outer protective layer.

ABSTRACT

A fiber optic cable includes a core and a surrounding protective layer. The core includes an inner tube having one or more optical fibers contained therein, and the surrounding protective layer includes an outer tube received over the inner tube, and a layer of buffer material positioned between the outer tube and the inner tube. The buffer material maintains the inner tube generally centrally located within the outer tube and providing a mechanical link between the inner tube and the outer tube to prevent relative movement therebetween. The inner tube may be coated with a low hydrogen permeability material to minimize the entrance of hydrogen into the inner tube. The low hydrogen permeability material may be coated with a protective layer of hard, scratch resistant material to protect the integrity of the low hydrogen permeability material. The area in the inner tube not occupied by the optical fibers may be filled with a filler material, the filler material being selected to have a sufficient viscosity to resist the shear forces applied to it as a result of the weight of the optical fibers within the tube while allowing movement of the optical fibers within the tube during spooling, deployment and handling of the cable to thereby prevent damage and microbending of the optical fibers. The filling material may be impregnated with a hydrogen absorbing/scavenging material to remove any excess hydrogen within the inner tube. The optical fibers have an excess length with respect to the inner tube, and the cable may include an outer jacket of a high temperature, protective material to protect the cable during handling and installation.

TECHNICAL FIELD

[0001] The present invention relates to fiber optic cables, and moreparticularly, to fiber optic cables for use in harsh environments.

BACKGROUND OF INVENTION

[0002] With advancements in the area of fiber optic sensors,particularly for use in harsh environments, such as in oil and gaswells, there is an increasing need for fiber optic cables that cansurvive harsh environments. For example, the harsh environmentencountered in down-hole fiber optic sensing applications placesdemanding requirements on the design of fiber optical cables for use inthe down-hole environment. Such a fiber optic cable may be used tointerconnect a down-hole fiber optic sensor with instrumentation locatedat the surface of a well bore.

[0003] Down-hole environmental conditions can include temperatures inexcess of 130° C., hydrostatic pressures in excess of 1000 bar,vibration, corrosive chemistry and the presence of high partialpressures of hydrogen. Down-hole applications also lead to therequirement that the fiber optic cable be produced in lengths of 1000 mand longer. Because of the long cable lengths in such applications, thefiber optic cable must be designed to support the optical fibercontained therein from excessive strain associated with the weight ofthe long length of optical fiber.

[0004] The deleterious effects of hydrogen on the optical performance ofoptical fiber, particularly in sub-sea installations for thetelecommunications industry, have long been documented. To protectoptical fibers from the effects of hydrogen, hermetic coatings andbarriers, such as carbon coatings and the like, have been used tominimize the effects of hydrogen in such sub-sea telecommunicationsapplications. However, at the elevated temperatures experienced in aharsh down-hole environment, such coatings lose their resistance topermeability by hydrogen. Additionally, at such high temperatures, theeffects of hydrogen on an optical fiber may be accelerated and enhanced.

[0005] Therefore, there exists the need for a fiber optic cable that issuitable for use in such harsh environments.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide a fiber opticcable for use in a harsh environment.

[0007] A further object of the invention is to provide such a fiberoptic cable that minimizes the exposure of optical fibers to hydrogencontained in the harsh environment, particularly at high temperatures.

[0008] A still further object of the invention is to provide such afiber optic cable wherein the optical fibers contained in the cable arenot exposed to significant damaging strain over a wide range ofoperating temperatures.

[0009] According to the present invention, a fiber optic cable includesa core and a surrounding protective layer. The core includes an innertube having one or more optical fibers contained therein, and thesurrounding protective layer includes an outer tube received over theinner tube, and a layer of buffer material positioned between the outertube and the inner tube, the buffer material maintaining the inner tubegenerally centrally located within the outer tube and providing amechanical link between the inner tube and the outer tube to preventrelative movement therebetween.

[0010] According further to the present invention, the inner tube may becoated with a low hydrogen permeability material to minimize theentrance of hydrogen into the inner tube. According still further to theinvention, the low hydrogen permeability material may be coated with aprotective layer of hard, scratch resistant material to protect theintegrity of the low hydrogen permeability material.

[0011] In still further accord with the invention, the area in the innertube may be filled with a filler material, the filler material beingselected to have a sufficient viscosity to resist the shear forcesapplied to it as a result of the weight of the optical fibers within thetube while allowing movement of the optical fibers within the tubeduring spooling, deployment and handling of the cable to thereby preventdamage and microbending of the optical fibers. According still furtherto the present invention, the filling material may be impregnated with ahydrogen absorbing/scavenging material.

[0012] According further to the invention, the optical fibers have anexcess length with respect to the inner tube. According further to theinvention, the cable may include an outer jacket of a high temperature,protective material to protect the cable during handling andinstallation.

[0013] The fiber optic cable of the present invention provides asignificant advantage over the prior art. The cable provides significantresistant to the damaging effects of hydrogen on an optical fiber byminimizing the exposure of the optical fibers to hydrogen. The innertube of the cable is coated with a low hydrogen permeability material tolimit the ingress of hydrogen into the inner tube. Additionally, thefilling material within the inner tube is impregnated with a hydrogenabsorbing/scavenging material to remove any hydrogen that may enter theinner tube. A protective coating is received over the low hydrogenpermeability material to maintain the integrity of the coating forhanding and manufacturing of the cable. To provide a high strength cablecapable of deployment in a harsh environment, the inner tube issurrounded by protective layer that includes a buffer materialsurrounded by an outer tube.

[0014] The foregoing and other objects, features and advantages of thepresent invention will become more apparent in light of the followingdetailed description of exemplary embodiments thereof, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a cross-sectional view of the fiber optic cable of thepresent invention; and

[0016]FIG. 2 is a perspective view of the fiber optic cable of FIG. 1within a well bore of an oil and/or gas well.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring now to FIG. 1, a fiber optic cable 10 manufactured inaccordance with the present invention includes a fiber in metal tube(FIMT) core 11 having an inner tube 13 surrounding one or more opticalfibers 16, 17. The inner tube 13 may be a laser welded tube, e.g., alength-wise laser welded tube, manufacture from a corrosion resistantmaterial, such as a corrosion resistant metal alloy. Examples ofsuitable corrosion resistant metal alloys include, but are not limitedto; Stainless Steel 304; Stainless Steel 316; Inconel 625; Incoloy 825.The inner tube 13 diameter may be in the range of 1.1 to 2.6 mm, and inan exemplary embodiment of the invention is 2.4 mm. Although the innertube is described as being 1.1 to 2.6 mm in diameter, the diameter ofthe inner tube may vary over a large range, depending upon the materialsused and the number of optical fibers to be placed in the inner tube.The inner tube 13 wall thickness is selected to be sufficient for thelaser welding process. For example, the inner tube 13 wall thickness fora Stainless Steel 304 tube may be 0.2 mm.

[0018] The inner tube 13 is coated or plated with a low hydrogenpermeability material coating 19, such as tin, gold, carbon, or othersuitable low hydrogen permeability material. The thickness of thecoating 19 is selected to provide a barrier to a high partial pressurehydrogen environment. Depending upon the selection of material, thecoating thickness may be in the range of 0.1 to 15 microns. For example,a carbon coating may have a thickness as thin as 0.1 microns, while atin coating may be approximately 1.3 microns in thickness. The coating19 may be over coated 21 with a protective layer of hard, scratchresistant material, such as nickel or a polymer such a polyamide. Theover coating 21 may have a thickness in the range of 2 to 15 microns,depending on the material.

[0019] The inner tube 13 may be filled with a filler material 22, togenerally fill the void spaces within the inner tube 13 not occupied bythe optical fibers 16, 17. The filler material 22 supports the opticalfibers 16, 17 within the inner tube 13. The filler material 22 isselected to have sufficient viscosity so as to resist the shear forcesapplied to it as a result of the weight of the fiber in a vertical wellinstallation to thereby provide the desired support for the opticalfibers 16, 17 over the entire operating temperature range of the cable10, including temperatures typically in the range of 10° C. to 200° C.,however, the cable may be used over a wider temperature range, dependingon the selection of materials, primarily related to the buffer material35 and coatings on the optical fibers 16, 17. Additionally, the fillermaterial 22 must allow the optical fibers 16, 17 to relax and straightenwith respect to the inner tube 13 due to differences in the coefficientsof thermal expansion between the optical fiber 16, 17 and the inner tube13 and during spooling and deployment of the cable 10. The viscosity ofthe filler material may widely vary, depending on the specific cabledesign, including the diameter of the inner tube and the number offibers in the inner tube. The filler material 22 also providesadditional benefits of preventing chaffing of the coatings on theoptical fibers 16, 17 as a result of bending action during installationand vibration of the cable 10. Another advantage is that the fillermaterial 22 serves as an integrator of inner tube surface roughness toavoid microbend losses in the optical fibers 16, 17. Suitable fillermaterials include standard thixotropic gel or grease compounds commonlyused in the fiber optic cable industry for water blocking, filling andlubrication of optical fiber cables.

[0020] To further reduce the effects of hydrogen on the optical fibers16, 17, the filler material 22 may be impregnated with a hydrogenabsorbing/scavenging material 23, such as palladium or tantalum.Alternatively, the inner surface 24 of the inner tube 13 may be coatedwith the hydrogen absorbing/scavenging material, or such material may beimpregnated into the tube material.

[0021] Referring also to FIG. 2, the cable 10 of the invention may beused in the wellbore 27 of and oil and/or gas well. The optical fibers16, 17 are selected to provide reliable transmission of optical signalsbetween the ends 25, 26 of the cable 10, such as between a fiber opticsensor 28 positioned within the wellbore 27 and optical signalprocessing equipment 30. Suitable optical fibers include low defect,pure silica core/depressed clad fiber. Alternatively, suitable fibersinclude germanium doped single mode fiber or other optical fibersuitable for use in a high temperature environment. Both fibers 16, 17may be of the same type or of different types. Although the invention isdescribed herein as using two optical fiber 16, 17 within the inner tube13, it will be understood by those skilled in the art that one or morefibers may be used. The total number of fibers within the inner tube 13is limited by the diameter of the inner tube such that sufficient spaceis provided within the inner tube to prevent microbending of the opticalfibers 16, 17 during handing and deployment of the cable 10.

[0022] The core 11 is surrounded by an outer protective layer 33 thatincludes a buffer material 35 and an outer tube 38. The buffer material35 provides a mechanical link between the inner tube 13 and the outertube 38 to prevent the inner tube 13 from sliding under its own weightwithin the outer tube 38. Additionally, the buffer material 35 keeps theinner tube 13 generally centered within the outer tube 38 and protectsthe inner tube and coating from damage due to vibration. Suitable buffermaterials include high temperature polymers, such asFluoroethylenepropylene (FEP), Ethylene-chlorotrifluoroethylene (ECTFE),Polyvinylidene fluoride (PVDF), perfluor alkoxy (PFA), TEFLON, TEFLONPFA, TETZEL, or other suitable materials. The buffer material 35 isfirst applied over the inner tube 13 after laser welding andcoating/plating, and then the outer tube 38 is welded over the buffermaterial and is either drawn down onto a compressible buffer material35, or the buffer material is expanded during a post laser weld thermalprocess. The outer tube 38 may be TIG welded, laser welded, or any othersuitable process for joining the outer tube 38 over the buffer material35 may be used. In the case of a compressible buffer material receivedbetween a 2.4 mm diameter inner tube and a 0.25 inch (6.345mm) outertube as illustrated in the exemplary embodiment of FIG. 1, the buffermaterial should have a thickness in the range of 0.183 inches (4.65 mm)and 0.195 inches (4.95 mm), and preferably 0.189 inches (4.80 mm).Although a range of buffer material thickness is described with respectto the exemplary embodiment of FIG. 1, any suitable thickness of buffermaterial may be used, depending of the dimensions of the inner tube andouter tube, to provide the desired mechanical protection of the innertube and/or to provide the mechanical linkage between the inner tube andthe outer tube to prevent relative movement therebetween.

[0023] The outer tube 38 is manufactured of a corrosion resistantmaterial that easily diffuses hydrogen. For example, the outer tube ismanufactured of the same material of the inner tube 13, without the lowhydrogen permeability coating or hydrogen scavenging material. The outertube 38 is provided in a standard diameter (after draw down ifapplicable), such as quarter-inch tubing (6.345 mm), and may have adiameter in the range of 4 to 10 mm. The outer tube 38 may have a wallthickness in the range of 0.7 to 1.2 mm.

[0024] The fiber optic cable 10 must be capable of operation over a widerange of temperatures, for example between 10° C. and 200° C. Inparticular, the cable must account for the differential thermalcoefficient of expansion (TCE) represented by the optical fibers 16, 17and the inner tube 13. Without accounting for the differential TCE, longterm stress of greater than 0.2% may be applied to the optical fibers16, 17 over the operating temperature range of the cable. Such stresscan lead to premature mechanical failure because of stress corrosion ofthe fibers 16, 17. To reduce the long-term stress applied to the opticalfibers 16, 17 as a result of installation into a high temperatureenvironment, the inner tube diameter is selected to be large enough tosupport an excess length or “serpentine over-stuff” of optical fiberwithin the inner tube 13. This excess length may be achieved bycontrolling the temperature rise of the inner tube material during laserwelding of the inner tube 13. The temperature is controlled such that itapproximates the anticipated maximum or normal operating temperature ofthe final installation. This process will lead to an excess length offiber within the inner tube upon cooling of the inner tube. An excesslength of up to 2.0% has been achieved using such method.

[0025] To further protect the cable 10 during handling and installation,a protective jacket 40 of a high strength, protective material may beapplied over the outer tube 38. For example, a jacket ofEthylene-chlorotrifluoroethylene (ECTFE) may be applied over the outertube 38 in a generally rectangular configuration to aid in the handlingand deployment of the cable 10. Other materials, such asFluoroethylenepropylene (FEP), Polyvinylidene fluoride (PVDF),Polyvinylchloride (PVC), HALAR, TEFLON PFA, or other suitable materialsmay be used as the protective jacket 40.

[0026] Although the invention has been described and illustrated withrespect to exemplary embodiments thereof, the foregoing and variousother additions and omissions may be made therein and thereto withoutdeparting from the spirit and scope of the present invention.

We claim:
 1. A fiber optic cable, comprising: an inner tube; one or moreoptical fibers positioned within said inner tube a layer of buffermaterial surrounding said inner tube; and an outer tube surrounding saidbuffer material; wherein said buffer material is positioned between saidinner tube and said outer tube and maintains said inner tube generallycentrally located within said outer tube.
 2. A fiber optic cableaccording to claim 1, wherein said buffer material limits relativemovement between said inner tube and said outer tube.
 3. A fiber opticcable according to claim 1, further comprising a coating on said innertube, said coating including a low hydrogen permeability material tominimize the entrance of hydrogen into said inner tube.
 4. A fiber opticcable according to claim 3, wherein said low hydrogen permeabilitymaterial is selected from the group consisting of tin, gold and carbon.5. A fiber optic cable according to claim 3, further comprisingprotective layer on said coating, said protective layer including ahard, scratch resistant material to protect the integrity of saidcoating of low hydrogen permeability material.
 6. A fiber optic cableaccording to claim 5, further comprising a filler material received insaid inner tube.
 7. A fiber optic cable according to claim 6, whereinsaid filler material is selected to have a sufficient viscosity toresist the shear forces applied to said filler material as a result ofthe weight of said optical fibers within said inner tube to generallymaintain the position of said optical fibers within said inner tube andto allow movement of said optical fibers within said inner tube duringmovement of the fiber optic cable.
 8. A fiber optic cable according toclaim 6, wherein said filler material is impregnated with a hydrogenscavenging material.
 9. A fiber optic cable according to claim 8,wherein said hydrogen scavenging material is selected from the groupconsisting of palladium or tantalum
 10. A fiber optic cable according toclaim 8, wherein said optical fibers have an excess length with respectto said inner tube.
 11. A fiber optic cable according to claim 9,further comprising an outer jacket of protective material surroundingsaid outer tube.
 12. A fiber optic cable according to claim 1, furthercomprising a filler material received in said inner tube.
 13. A fiberoptic cable according to claim 12, wherein said filler material isselected to have a sufficient viscosity to resist the shear forcesapplied to said filler material as a result of the weight of saidoptical fibers within said inner tube to generally maintain the positionof said optical fibers within said inner tube and to allow movement ofsaid optical fibers within said inner tube during movement of the fiberoptic cable.
 14. A fiber optic cable according to claim 12, wherein saidfiller material is impregnated with a hydrogen scavenging material. 15.A fiber optic cable according to claim 14, wherein said hydrogenscavenging material is selected from the group consisting of palladiumor tantalum
 16. A fiber optic cable according to claim 1, wherein saidoptical fibers have an excess length with respect to said inner tube.17. A fiber optic cable according to claim 1, further comprising anouter jacket of protective material surrounding said outer tube.
 18. Afiber optic cable according to claim
 19. A fiber optic cable accordingto claim 1, wherein said filler material is selected from the groupconsisting of Fluoroethylenepropylene (FEP),Ethylene-chlorotrifluoroethylene (ECTFE), Polyvinylidene fluoride(PVDF), perfluor alkoxy (PFA), TEFLON, TEFLON PFA and TETZEL.